Method of establishing markers in boreholes



Patented Apr. 24, 1951 METHOD OF ESTABLISHING DIARKERS IN s BOREHOLESHenri-Georges Doll, Houston, Tex., assignor to Schlumberger WellSurveying Corpora-tion, Houston, Tex., a corporation of DelawareApplication December 22, 1943, Serial No. 515,235

7 Claims. l

This invention relates to improved methods for establishing detectablereference points in the cased section of a bore hole in known relationto a formation or to another depth marker disposed outside the metalliccase.

The desirability of establishing reference depth markers in bore holesis pointed out, and methods and apparatuses utilizing such markers aredisclosed in the Doll application Serial No. -i-3,30, now Patent No.2,475,137 granted July l2, 1949, filed May 16, 1942, and the Segesmanapplication Serial No. 379,983, now Patent No. 2,350,832, granted June6, 1944. The principal use of these markers is to facilitate the placingof a gun perforator in a bore hole very accurately with respect to astratum, particularly one at greater depth.

In order to position the perforator accurately, it is necessary to lowerthe perforator and the marker detecting means into the bore holesimultaneously on the same cable. Gun perforators require relativelylarge powder charges to propel their projectiles through the steelcasing and enormous shock and vibration, therefore, accompany the firingof a perforator.

The shock conditions during ci-lring of the pertorator are destructiveand unless the marker locating device is especially constructed towithstand shock, it may be destroyed, or at least partially damaged. Theeconomic factors involved preclude the use of a new marker locatingdevice for each perforating operation and heretofore the locatingdevices have been constructed. at a very considerable expense,to'withstand the shock encountered during firing of the gun perforator.

One ci the least expensive and most reliable markers that can beprecisely detected through the steel casing is a radio-active markerthat spontaneously` gives oli gamma-rays, which rays easily penetratethe casing. Gamma-ray detectin g a pparatus is inherenimlitifad o/stlyand, acoordinglbann'otfbeattached to a gun perforator unless specialprecautions, involving considerable expense, are taken to prevent damageto the apparatus. Similar diiculties are encountered in the use of othermarker locating apparatuses.

It is an object of this invention to provide arnethod for establishingand locating a marker in the Casin;r which can be detected by means ofsturdy and inexpensive equipment.

Another object of the invention is to provide a method of establishingand locating a depth marker in a casing by means of equipment which isneither delicate noiyexpensive.

lll

A further object of the invention is to provide a method for marking thebore hole casing at any desired depth in such a manner that the markedportion of the casing can be readily detected by lowering a suitableapparatus into the casing either alone or in combination with a gunperforator.

Other objects of the invention will become apparent from the followingdetailed description of a typical embodiment of the present invention.

In accordance with the present invention, an easily detectable markercan be established in a cased bore hole by lowering an electrode intothe casing and passing a rather high direct current between the casingand the electrode, thereby polarizing a short section of the casing.

The polarized section of the casing will retain its polarity for aconsiderable length of time, usually considerably longer than isrequired for relocating the marker for subsequent operations in the borehole. The presence of the polarized marker can be readily detected byany one of a number of known types of apparatus which can be used aloneor in combination with a gun perforator, whereby the gun perforator canbe accurately positioned in the casing or other determinations made inthe bore hole.

In establishing the electrical or polarized marker in the casing, aradio-active marker may be used as a reference point. This marker may bepositioned in a formation of interest before the casing is inserted. Bylowering a radiationdetecting device and the polarizing electrode, inknown spatial relationship, into the casing until the detecting deviceis opposite to the radioactive marker, the polarized marker can beestablished in the casing in a. known relation to the radio-activemarker.

Sturdy and inexpensive equipment can then be utilized to locate thepolarized marker and to position a perforating gun accurately withrespect to the formation of interest.

For a better understanding of the present invention, reference may behad to the accompanying drawings in which:

Figure 1 is a view in longitudinal section through a bore hole casingillustrating, schematically, apparatus for producing and detecting anelectrical depth marker;

-Figure 2 is a partial longitudinal section through the bore hole andapparatus illustrating the method of locating a marker positionedexternally of the casing and then establishing an electrical markerwithin the casing in known depth relation thereto:

Figures 3A, 3B, 3C and 3D are reproductions KUSS Ktf'tiitNCl;

of the natural potential curves recorded at various time intervals afterproducing an electrical depth marker; and

Figure 4 is a curve showing the exponential decay as a function of timeof the natural potentials produced by an electrical marker.

According to the present invention, an electrical depth marker may beestablished at any place in the cased portion oi the bore hole bylowering an electrode into the hole on an insulated electric cable, suchas a conventional well logging cable. When at the desired position, arelatively heavy direct current is caused to ow between the electrodeand the casing for a short length of time. This produces a polarizationalong a short length of the casing at the depth of the electrode. Thiszone of polarization may then be very easily detected by conventionalpotential measurements. Apparatus such as that shown in SchlumbergerPatent No. 1,913,293 is suitable for this purpose.

Referring to Figure l, a bore hole I containing a column of liquid 1I isshown in longitudinal section. A conventional steel casing I2 isinserted in the bore hole. An electrode I3 is suspended in the bore holeon an insulated electrical conductor I4, the upper end of which isconnected to a switch I8. When the switch is thrown to the right, theelectrical conductor is connected through an ammeter I5 and a source ofdirect current I6 to the casing or ground as at I7. In the otherposition of switch I8, the electrical cable. I4 is connected through agalvanometer or other potential measuring instrument I9 to the casing asVat I'I.

In operation the electrode i3 is lowered into the bore hole to thedesired depth on a cable I4 by means of a winch or other suitablemechanism which is not shown. When the electrode is at the depth atwhich it is desired to establish an electrical marker, the switch I8 isthrown to the right which causes a direct curr-ent to ow from thebattery I6, through ammeter I5, through electrical conductor I4 and fromelectrode I3 to the casing I2 through which the circuit is completed tothe surface. As will be explained in more detail subsequently, it hasbeen found in practice that a current of four or ve arnperes owing forten minutes is sucient to establish an easily detectable marker whichmay be 1o cated several days after it is established.

Figure 2 is a section through a portion of a bore hole schematicallyillustrating the method for positioning an electrical marker in a knownspatial relationship with a particular formation of interest, or amarker which has been placed outside the casing. Referring to Figure 2 aradio-active marker 20 is placed in known relation to a stratum ofinterest, for example, in a thin oil sand before the casing I2 isinserted in the bore hole. `To create an electrical marker within thecasing in known relation to the marker 20, a gamma-ray detectingapparatus 2l is lowered into the bore hole on an electrical cable 24. Anelectrode 22 is positioned on/or adjacent the assembly 2I and connectedelectrically to the surface by any suitable means, such as conductor 23.The assembly is raised or lowered in the bore hole until the position ofmarker 2U is definitely established by the assembly 2i by any of themethods well known in the art. Direct current is then caused to flowbetween the electrode 22 and the casing I2. By knowing the distancebetween the electrode 22 and the effective point of measurement of thegamma-ray 4 detecting assembly 2|, the relative depth of the electricalmarker may be established.

Figures 3A, 3B, 3C and 3D are reproductions of natural potential logswhich have been produced by conducting the method in a cased bore hole,the operations being conducted at the depth indicated on the graphs. Inthe particular field operation which has been here shown for the purposeof illustration, a. direct current of four amperes was sent between theelectrode and the casing for ten minutes and the curve of Figure 3Arecorded ve minutes after cutting off the current. Curves 3B, 3C and 3Dwere recorded thirty minutes, forty-five minutes and one and a halfhours, respectively, after interrupting the current. It can thus be seenthat the electrical anomaly produced is very sharp and persists for areasonable length of time. It will also be noted that the indication ofthe presence of the marker is very great compared to the background orspurious potentials present in the casing.

Figure 4 is a curve plotted from the data of Figures 3A through 3D andadditional data taken at longer intervals. As shown in Figure 4, themaximum natural potential adjacent the marker has been plotted againsttime and it will be noted that the maximum value decays exponentiallywith time. It may thus be seen that this type of electrical marker maybe easily located several days after it is established. Should it bedesired to have the marker persist for a longer interval, a. greatercurrent could be used for a greater length of time when producing theelectrical marker,

Field tests have indicated that it is relatively unimportant whichdirection the current ows when producing the electrical marker A.Electrical markers have been successfully established by flowing thecurrent in each direction. When the direction of current flow isreversed, the direction of the peak on the voltage curves, or itspolarity, is reversed, but the marker indication remains sharp.

If desired, the body of the perorating gun, or part of the body, may beused as an electrode instead of providing an additional one. Mostelectrically fired gun perforating equipment is already provided withmeans or denoting the variations in potential as this is very useful infollowing the lowering of the perforator into the hole. While thespurious potentials present in the casing are not large, they aresufficient to check to see whether the gun periorator is moving. Thus itmay be seen that this particular type of depth marker may be located byconventional electrically operated gun perforating equipment as it isused without any additions. The marker may be placed with conventionalelectrical loeging equipment as it is used and does not require anyadditional expense for either positioning or locating the marker.

It will be apparent from the foregoing that the invention provides asimple, effective and fast method for positioning a gun periorator orother tool in a bore hole in known relation to a stratum of interest orother marker placed outside the casing or to locate a marker in the borehole to permit making consistent depth measurement-S therein, the markernot necessarily being placed in any particular relation to a stratum ormarker lying outside the casing.

Although this specic embodiment of the invention has been described indetail, the invention is not to be limited thereto, but is susceptiblestarrte toom of numerous changes within the scope of the appendedclaims.

I claim:

l. A method of establishing a detectable depth marker in a casing in abore hole in known relationship to radioactive material outside thecasing, comprising lowering into the bore hole a detector for saidradioactive material outside said casing and an electrode in knownspatial relationship to said detector, stopping said electrode in apredetermined relation to said radioactive material outside the casing,and passing a direct current between the casing and said electrode whilesaid electrode is stopped in said piedetermined relation to theradioactive material outside said casing.

2. A method of establishing a detectable depth marker in a casing in abore hole in known relationship to a radiation-emitting marker outsidesaid casing, comprising lowering into said bore hole a detector for saidradiation-emitting marker and an electrode in known spatial relation tosaid detector, and passing a direct current between said casing and saidelectrode to polarize a portion of said casing while said detector andelectrode are stopped.

3. A method of establishing a detectable depth marker in a. casing in abore hole in known relationship to radioactive material outside thecasing, comprising locating said radioactive material by lowering adetector therefor into the casing, lowering an electrode into saidcasing, stopping the electrode adjacent a portion of the casing at azone where a depth marker is to be established, passing a direct currentbetween said electrode :and said casing to polarize said casing portion,and determining the relative positions of said polarized casing portionand said radioactive material outside the casing.

4. A method of establishing a detectable depth marker in a casing in abore hole in known relationship to a marker outside the casing,comprising locating said marker outside the casing by lowering adetector therefor into the casing, lowering an electrode into saidcasing, stopping the electrode adjacent a portion of the casing at azone where a depth marker is to be established, passing a direct currentat about 4 to 5 amperes value between said electrode and said casing topolarize said casing portion, and determining the relative positions ofsaid polarized casing portion and said marker outside the casing.

5. A method of establishing a detectable depth marker in a casing in abore hole in known relationship to a marker outside the casing,comoutside the casing.

6. A method of establishing a detectable depth marker in a casing in abore hole in known relationship to a marker outside the casing,comprising lowering into the bore hole a detector for said markeroutside said casing and an electrode in known spatial relationship tosaid detector, stopping said electrode in a predetermined relation tosaid marker outside said casing, and passing a direct current at about 4to 5 amperes value between the casing and said electrode while saidelectrode is stopped in said predetermined relation to the markeroutside said casing.

7. A method of establishing a detectable depth marker in a casing in abore hole in known relationship to a marker outside the casing,comprising lowering into the bore hole a detector for said markeroutside said casing and an electrode in known spatial relationship tosaid detector, stopping said electrode in a predetermined relation tosaid marker outside said casing, and passing a direct current at about 4to 5 amperes value for about ten minutes between the casing and saidelectrode while said electrode is stopped in said predetermined relationto the marker outside said casing.

HENRI-GEORGES DOLL.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date Re. 19,611 DeForest June 18, 19352,085,664 Karcher June 29, 1937 2,141,826 Schlumberger Dec. 27, 19382,165,013 Schlumberger July 4, 1939 2,183,565 Hawley Dec. 19, 19392,228,623 Ennis Jan. 14, 1941 2,259,904 McNamee et al. Oct. 21, 19412,273,363 Lipson Feb. 17, 1942 2,297,754 Ennis Oct. 6, 1942 2,322,797Hume Oct. 26, 1943 2,371,658 Stewart Mar. 20, 1945

